As well known in the art, the tensions in the warp yarns forming a shed in a weaving loom increase temporarily during beating operation in which the pick of weft yarn which has been inserted into the shed during each cycle of operation of the loom is beaten up onto the edge or "fell" of the woven fabric at the trailing end of the fabric which is being produced. It is further known that the degrees of variation in the tensions produced in the warp yarns during beating operation are greater in those warp yarns which are engaged by the weft yarn being beaten up than those warp yarns, if any, with which the weft yarn being beaten up has failed to engage. An improper pick detecting device has therefore been proposed and put into use for detecting a pick of weft yarn which has failed to be inserted throughout the width of the weaving shed is detected by monitoring the tensions in several to tens of warp yarns forming part of the shed. The tensions in these warp yarns are detected by means of a tension sensing unit which is arranged to be constantly in contact with the particular warp yarns for producing an analog electric signal continuously variable with the total of detected warp yarn tensions during weft beating operation in each cycle of operation of the loom. The analog signal thus produced by the individual tension sensing unit is compared with a reference signal which is representative of the sum of warp yarn tensions to be produced when the warp yarns are engaged during beating operation by a pick of weft yarn which has failed to reach the farthest end of the weaving shed but which has been inserted into the shed to a length covering most of the warp yarns or, more exactly, a practically allowable minimum proportion of the total width of the shed. If the signal representative of the warp yarn tensions detected during beating operation in a certain weaving cycle of the loom is proved to be lower in magnitude than such a reference signal, the improper pick detecting device delivers an output signal indicating that the pick of the weft yarn which has been inserted into the shed during the particular weaving shed is improper, or has failed to cover the allowable minimum proportion of the total width of the shed. The output signal of the improper pick detecting device is used typically for causing the loom to a stop or producing a warning signal so as to enable the operator of the loom to service or troubleshoot the loom as the weft-filling stage of the loom.
When the weaving operation is interrupted in the process of producing a woven fabric, a "stop mark" tends to be formed in the barbic and impairs the commercial value of the fabric completed. In order to minimize the number of the stop marks to be produced in a woven fabric to be manufactured by a loom equipped with an improper pick detecting device of the above described nature, it is desirable that the value represented by the reference signal to be produced in the device be set to be as small as possible. If the reference signal is reduced in magnitude to achieve such a purpose, then the improper pick detecting device would objectionably respond to the variation in the warp yarn tensions which increase as the diameters of the rolls of the warp yarns on a warp beam of the loom vary as well known in the art.
In another type of prior-art improper pick detecting device used in a weaving loom, a signal representative of the warp yarn tensions detected during beating operation in each cycle of operation of the loom is memorized and utilized for producing a reference signal to be used in the immediately subsequent cycle of operation. The reference signals thus produced are practically free from the effects of the above mentioned cyclic variation in the warp yarn tensions but are, instead thereof, affected by the constructive interference between the waves of vibration of the detecting device and the support for the detecting device as will be explained in greater details.
In a dobby loom in which the weaving shed is formed by three or more sets of warp yarns in each cycle of operation of the loom, the tensions in the warp yarns not only increase as above described but vary from one cycle of operation to another of the loom because each of the weft yarns to form each of the complete weaves (to be defined later) of the fabric to be woven is contacted by the warp yarns which are arranged differently from the warp yarns to be contacted by the other weft yarns in each complete weave.
All these problems encountered in prior-art improper pick detecting devices of the described types are ascribed to the lack of reliability of the reference signals used in such devices.
An important object of the present invention is, therefore, to provide a method of and a device for detecting an improper pick of weft yarn in a weaving loom by producing a reference signal which is not susceptible to the effects of extraneous factors which are inherent in the loom.
It is another important object of the present invention to provide a method of and a device for detecting the occurrences of improper picks of weft yarn in a weaving loom without the risk of causing frequent and/or objectionalbe interruptions of the weavinf operation in the process of manufacturing a woven fabric in the loom.
Yet, it is another important object of the present invention to provide a method of and a device for detecting an improper pick of weft yarn in a weaving loom accurately and reliably regardless of the periodic variation in the tensions in the warp yarns forming a shed in the loom.
It is still another important object of the present invention to provide a method of and a device for detecting an improper pick of weft yarn in a weaving loom without being affected by the mechanical vibrations to be produced in the loom during operation.